Recently, there has been an increasing interest in energy storage technology. Batteries have been widely used as energy sources in the fields of cellular phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development into them. In this regard, electrochemical devices are one of the subjects of great interest. Particularly, development of rechargeable secondary batteries has been the focus of attention. Recently, the research and development into a novel electrode and a novel battery that can improve capacity density and specific energy have been made intensively in the field of the secondary batteries.
Among currently used secondary batteries, lithium secondary batteries developed in early 1990's have a higher drive voltage and a much higher energy density than those of conventional batteries using a liquid electrolyte solution such as Ni-MH batteries, Ni—Cd batteries, and H2SO4—Pb batteries. For these reasons, the lithium secondary batteries have been advantageously used. However, such a lithium secondary battery has disadvantages in that organic electrolytes used therein may cause safety-related problems such as ignition and explosion of the batteries and that processes for manufacturing such a battery are complicated. Recently, lithium-ion polymer batteries have been considered as one of the next-generation batteries since the above disadvantages of the lithium ion batteries are solved. However, the lithium-ion polymer batteries have a relatively lower battery capacity than those of the lithium ion batteries and an insufficient discharging capacity in low temperature, and therefore these disadvantages of the lithium-ion polymer batteries remain to be urgently solved.
Such electrochemical devices have been produced from many companies, and the battery stability has different phases in the electrochemical devices. Accordingly, it is important to evaluate and ensure the stability of the electrochemical batteries. First of all, it should be considered that errors in operation of the electrochemical device should not cause damage to users. For this purpose, the Safety Regulation strictly regulates ignition and explosion in the electrochemical devices. In the stability characteristics of the electrochemical device, overheating of the electrochemical device may cause thermal runaway, and explosion may occur when a separator is pierced. In particular, a polyolefin porous substrate commonly used as a separator of an electrochemical device shows extreme thermal shrinking behavior at a temperature of 100° C. or above due to the features of its material and its manufacturing process such as elongation, so there may occur an electric short circuit between cathode and anode.
In order to solve the above safety-related problems of the electrochemical device, there has been proposed an organic/inorganic composite separator having a porous active layer formed by coating at least one surface of a polyolefin porous substrate having many pores with a mixture of inorganic particles and a binder polymer (see Korean Laid-open Patent Publication No. 10-2006-72065 and 10-2007-231, for example). The inorganic particles in the porous active layer formed on the polyolefin porous substrate act as a kind of spacer that keeps a physical shape of the porous active layer, so the inorganic particles restrain thermal shrinkage of the polyolefin porous substrate when the electrochemical device is overheated. In addition, interstitial volumes exist among the inorganic particles, thereby forming fine pores.
As mentioned above, at least a certain amount of inorganic particles should be contained such that the porous active layer formed on the organic/inorganic composite separator may restrain thermal shrinkage of the polyolefin porous substrate. However, as the content of inorganic particles is increased, a content of binder polymer is relatively decreased, which may cause the following problems.
First, due to the stress generated in an assembly process of an electrochemical device such as winding, inorganic particles may be extracted from the porous active layer, and the extracted inorganic particles act as a local defect of the electrochemical device, thereby giving a bad influence on the stability of the electrochemical device.
Second, an adhesion between the porous active layer and the polyolefin porous substrate is weakened, so the ability of the porous active layer to restrain thermal shrinkage of the polyolefin porous substrate is deteriorated. Thus, it is difficult to prevent an electric short circuit between cathode and anode even when the electrochemical device is overheated.
On the contrary, if the content of binder polymer in the porous active layer is increased in order to prevent extraction of inorganic particles, the content of inorganic particles is relatively decreased, so thermal shrinkage of the polyolefin porous substrate may not be easily restrained. Accordingly, it is hard to prevent an electric short circuit between cathode and anode, and also the performance of the electrochemical device is deteriorated due to the decrease of porosity in the porous active layer.